In prior art lighting devices it is known to position one or more light sources behind a screen such that the observer cannot distinguish the individual light emitting light sources. The most obvious example is a diffusing screen or sphere that obscures the direct image of the light emitting light sources.
Nowadays, with the increased use of LEDs in lighting devices a direct view of the light source is hardly ever appreciated and masking LED sources is generally an integral part of the development of LED-based lighting devices. Using diffusers in a manner similar to how it was done in connection with other light sources, such as light bulbs, is known.
However, such a straightforward implementation suffers from several drawbacks. For a diffuser to effectively mask the sources the distance to the sources should be comparable to or larger than the pitch of the LEDs. The luminaire form factor is thus largely determined by this LED masking. As LEDs become more efficient, the number of LEDs is reduced and as a result the pitch increases. It is desirable that the luminaire form factor is independent of the LED pitch. In addition, in case of LED failure a dark area on the diffusing screen will appear, in which case the whole lighting module needs to be replaced.
Furthermore, a high degree of collimation is required particularly for spotlight applications. A diffusing screen, however, modifies and even limits the degree of collimation required for spotlights, and hence is not considered an ideal solution to mask the individual sources.
US 2006/0291203 A1 describes an attempt at solving the above problems by means of a light mixing module in which a red, a green and a blue LED is arranged in a reflective small package, the light emitted from the LEDs being reflected from the walls of the small package and thus being mixed. Further mixing is obtained by means of a fiber bundle arranged extending from the light exit surface of the small package.
Although the individual light sources are thereby masked, this solution, however, still gives the viewer the impression of several light sources as each fiber of the fiber bundle provides a separate light spot. Furthermore, this solution does not solve the problem of providing collimation of the light in a degree sufficient for instance for spot light applications.
Therefore there is a need to provide a lighting device emitting light with a luminance that gives the observer the impression that only one single light source is present in the lighting device. Furthermore it is preferred that the user may choose a light source having a shape or form of his liking, for instance a uniform disc or an o-ring shape. This should be done in such a way that the image of the source is independent of the actual number of light sources, preferably LEDs, present in the lighting device. Also, the form factor should preferably be independent of the LED pitch and/or be minimized and the degree of collimation should preferably not be compromised or be compromised minimally.